Description of the Prior Art
Disk drives for storing data include one or more magnetic heads or transducers that are moved during an accessing or seek mode to selected data tracks by means of a head actuator, such as a voice coil motor. During the seek mode, the head actuator follows a position profile that is stored in a microprocessor, by way of example. The velocity profile is used to compare the actual position of the head during accessing to the desired track position provided by the microprocessor.
As disk drive technology progresses, disk drives are made more compact and the spacing between the concentric data tracks are reduced to afford high track density and increased data storage capability. Servosystems are used to ensure that the heads accesses the selected data tracks in minimal time and that the head follows the center of the selected track very closely so that data will be properly recorded and read out. Prior to actual read/write (R/W) operation of the disk drive, a pattern of servo information signals are prerecorded or embedded on the surface of the magnetic disk. Typically the servo signals are recorded in a plurality of spaced sectors interleaved between the data tracks along the length of the concentric tracks. In a well known implementation of a servosystem used with disk drives, the servo field is formed in a checkerboard pattern of transitions which are aligned radially but are of opposite polarity at alternate tracks. The servo field comprises a quadrature pattern that is aligned with the data tracks. The interleaved tracks of the servo field are arranged so that the boundary between adjacent servo tracks is at the centerline of the data track between the servo tracks. By sensing the servo information with the R/W head, an indication of the position of the R/W head relative to the nearest track center is obtained and an on-track or off-track condition can be detected.
Each data track on the surface of the disk carries a personalized address mark to identify the tracks during the seek mode. The address mark may consist of three or more bytes having different combinations for each track. A problem that is encountered during the seek mode in high track density drives is the occurrence of noise and interference which results from the rapid changes of a plurality of bits of the address mark as the head moves radially across the tracks. As a result the information being read out can be erroneous. The circuitry for processing the address information and the servo information generally is relatively complex and costly to design, construct and maintain.
Prior art servosystems are generally limited to displacements from a selected track of +/-30% from the position profile, in relation to the number of data tracks, to be capable of proper accessing and track following of the track. Therefore if the head or transducer deviates in position beyond such limit, useful read or write functions cannot be implemented.